The advent of a new round of stringent emissions legislation in Europe and North America is driving the implementation of new exhaust after-treatment systems, particularly for lean-burn technologies such as compression-ignition (diesel) engines, and stratified-charge spark-ignited engines (usually with direct injection) that are operating under lean and ultra-lean conditions. Lean-burn engines exhibit high levels of nitrogen oxide (NOx) emissions that are difficult to treat in oxygen-rich exhaust environments characteristic of lean-burn combustion. Exhaust after-treatment technologies are currently being developed that will treat NOx under these conditions. One of these technologies comprises a catalyst that facilitates the reactions of ammonia (NH3) with the exhaust nitrogen oxides (NOx) to produce nitrogen (N2) and water (H2O). This technology is referred to as Selective Catalytic Reduction (SCR).
Ammonia is difficult to handle in its pure form in the automotive environment. Therefore, it is customary with these systems to use a liquid aqueous urea solution, typically at a 32% concentration of urea solution (CO (NH2)2). The solution is referred to as AUS-32, and is also known under its commercial name of AdBlue. The urea solution is delivered to the hot exhaust stream and is transformed into ammonia in the exhaust after undergoing thermolysis, or thermal decomposition, into ammonia and isocyanic acid (HNCO). The isocyanic acid then undergoes a hydrolysis with the water present in the exhaust and is transformed into ammonia and carbon dioxide (CO2). The ammonia resulting from the thermolysis and the hydrolysis then undergoes a catalyzed reaction with the nitrogen oxides as described previously.
At high temperatures, AUS-32 or AdBlue can decompose into other chemical compounds, including, but not limited to, biuret, melamine, and others. These compounds can exhibit properties of a hard, clay-like substance that is not soluble in water. Below a certain threshold temperature, these deposits can build up to the point of obstructing the injected urea solution spray, resulting in a malfunction of the exhaust after-treatment system.
A conventional RDU is mounted on the exhaust manifold with two mounting screws. The mounting boss on the exhaust manifold receives the RDU, with a gasket between the injector flange and the boss mating surfaces. The gasket material currently used is non-asbestos fiber. Testing has shown that under certain conditions, urea solution decomposition by-products will form on the surfaces defining the gasket through-hole. Once these gasket deposits form, they tend to “grow”, forming a buildup that extends to the exhaust boss through-hole surfaces and then beyond into the exhaust space. In some cases, these deposit growths may progress to obstruct the through-hole entirely.
Thus, there is also a need to provide a shield associated with a gasket of an RDU/exhaust boss interface that minimizes the build-up of urea solution deposits on the gasket.